焼岳天皇泉うぐいすの湯1号泉Yakedake Tenno-sen Uguisu-no-yu Spring No. 1
A sodium-led bicarbonate spring with a chloride layer — the classic juso-sen style, soft on the skin at a neutral pH. A generous 219 mg/kg of metasilicic acid puts it squarely in Japan's beauty-water family, leaving the kind of silica film skin notices afterwards. At the source the water is almost colorless and clear, yet it tastes salty with faint sulfur, iron and carbonic notes and carries a hydrogen sulfide smell. The certificate samples it at the bath inlet of the Uguisu-no-yu pool, after the water has been cooled through the property's heat exchanger.
Good for
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, netto 熱の湯).
cold sensitivity
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺526mgP78 national68%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺115mgP77 national17%
- Magnesiummild; a laxative if you drink itMg²⁺33.7mgP78 national8%
- Potassiumminor; matters for drinking limitsK⁺36.1mgP80 national3%
- AmmoniumNH₄⁺14.8mgP96 national2%
- LithiumLi⁺3.5mgP93 national1%
- StrontiumSr²⁺1.5mgP84 national0.1%
- Iron (II)Fe²⁺0.80mgP60 national0.1%
- BariumBa²⁺0.80mgP85 national0.0%
- ManganeseMn²⁺0.10mgP36 national0.0%
- AluminumAl³⁺0.02mgP22 national0.0%
- CopperCu²⁺0.02mgP78 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,430mgP94 national70%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻356mgP70 national30%
- FluorideF⁻1.1mgP47 national0.2%
- SulfateSO₄²⁻1.8mgP10 national0.1%
- BromideBr⁻1.7mgP71 national0.1%
- Hydrogen sulfideHS⁻0.40mgP50 national0.0%
- IodideI⁻0.40mgP66 national0.0%
- Hydrogen phosphateHPO₄²⁻0.06mgP22 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃219mgP89 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂58.8mgP91 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂362mgP93 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.90mgP74 national
- ArsenicAsNDmg
- CadmiumCdNDmg
- ChromiumCrNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
Measured at the source
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.